I may just be ignornant but isn't hyperthyoridsm associated with the unability to gain weight, or to be extact sudden drops in weight, not entirely sure.

I also heard that hypo/hyper alter your BMR but even severe cases lower it by 200-300, once again don't quote me on that, simply just heard it from other people. Are you monitoring what your client is eating, because I don't think anyone is going to lose weight if they stick to old eating habbits and just starting lifting.